Organic electroluminescence (EL) devices are capable of obtaining a light emission with high luminance intensity at low voltage, and thus, have gained attention as a promising display device. An important characteristic value of the organic electroluminescence device is electric power consumption. The electric power consumption is expressed as the product of voltage and electric current, and the lower the voltage is required to obtain a desired brightness and the smaller value the electric current becomes, the lower the power consumption of the device may be made.
As one attempt to lower the value of electric current that flows through the device, a luminescence device utilizing light emission from an ortho-metalated iridium complex (Ir(ppy)3: tris-ortho-metalated complex of Iridium(III) with 2-phenylpyridine) has been reported (see, for example, Patent Documents 1 to 3). The phosphorescent luminescence device described therein is greatly enhanced in the external quantum efficiency as compared with singlet luminescence devices of the related art, and thus, succeeded in lowering the value of electric current.
As described above, although the iridium complex is used as a light emission material for implementing light emission efficiency, the solubility is generally poor, and thus, there was a case where the service life of the device was shortened due to aggregation or association of the complex. In particular, when the complex has a condensed ring structure such as a quinoline ring and an isoquinoline ring in the ligand, it is considered that the degree of freedom of molecular motion is decreased, and thus, the complex is easily aggregated. In order to suppress aggregation or association of the complex, an attempt to increase the solubility has been made by introducing, for example, a specific substituent (Patent Documents 4 and 5), but the effects thereof were not sufficient.
Meanwhile, devices using a compound which has an indolocarbazole structure as a host material for the purpose of improving light emission efficiency and durability of a phosphorescent luminescence device (Patent Documents 6 and 7) have been reported, but more improvements are demanded from the viewpoint of durability and light emission efficiency.
Further, since indolocarbazole also has an extended p-conjugated plane, and has a small degree of freedom of molecular motion, it is considered that association and aggregation is easily generated.
In addition, in the manufacture of an organic electroluminescence device, a method for forming a thin film which is an organic layer disposed between a pair of electrodes includes a deposition method such as vacuum deposition, and a wet method such as a spin coating method, a printing method and an inkjet method.
Among them, if a wet method is used, it is also possible to use polymeric organic compounds, for which it is difficult to form a film in dry processes such as vapor deposition, and when a wet method is used in a flexible display and the like, the method is appropriate from the viewpoint of durability such as flex resistance, film strength and the like, and when the device is manufactured to have a large area, the method is particularly preferable.
However, organic electroluminescence devices obtained by a wet method were problematic in that the devices have low light emission efficiency or device durability.